The invention relates generally to a system and method for the efficient storing and discharging of hydrogen in a hydrogen storage system.
Hydrogen is currently considered as a potentially useful energy source. One of the challenges in utilizing hydrogen has been the need for simple, efficient, reliable and cost-effective techniques for compressing, storing and releasing hydrogen.
It is generally known that hydrogen may be stored in compressed gaseous form in pressure vessels or at low temperature as liquid hydrogen. It is also known that hydrogen may be stored in storage vessels that contain powdered metal hydrides. The collection of metal hydrides in a storage vessel is referred to as a bed. Metal hydrides comprise metal atoms that constitute a host lattice. The host lattice traps hydrogen atoms in interstitial sites, such as lattice defects. Storage of hydrogen as a solid hydride provides greater volumetric storage density compared to that of compressed hydrogen gas or liquid hydrogen in pressure vessels. However, in such solid hydride systems, an important consideration is the ability to repeatedly charge and discharge hydrogen while retaining the hydrogen storage capability of the storage vessel.
The hydrogenation process of metals and metallic alloys is an exothermic process. The heat released during hydrogenation or absorption of hydrogen storage alloys must be removed in order to store the hydrogen in the metal hydrides. Ineffective heat removal can cause the hydrogenation process to slow down or terminate. Unless heat is effectively removed from the metal hydride powder when hydrogen is added to the storage vessel, it may not be practical to fill the storage vessel with usable hydrogen in a reasonably short time period. Consumers will not be motivated to use hydrogen as a fuel if it takes an unreasonably long time to refuel the on-board hydrogen storage vessel of the vehicle. Thus, effective heat removal from the metal hydride powder during hydrogenation is an important aspect of being able to practically use hydrogen for many applications such as a fuel for vehicles.
A related issue is that heat also needs to be generated to release stored hydrogen when the hydrogen fuel stored in a storage vessel is used. This process may also be referred to as dehydrogenation or desorption. For these reasons, effective thermal management of the hydrogen storage material is an important aspect related to the reliable hydrogenation and dehydrogenation of metal hydrides or other storage materials.
In typical systems, hydrogen storage vessels are coupled with heat exchangers or the like to cool the metal hydride bed during charging. In this process, cooling water or the like is circulated into the hydrogen storage tank to cool the metal hydride bed, thereby providing an increase in the amount of hydrogen absorbed. The heat exchanger may also be employed to introduce heat into a hydrogen storage vessel to begin the process of dehydrogenation or desorption.
Conventional systems may include heat sinks or heat distribution fins thermally coupled to the heat exchanger or hydrogen storage vessel to dissipate heat during hydrogenation and dehydrogenation. Heat dissipation may also be facilitated using heat pipes. Thus, typical hydrogen systems rely on heat conduction as a primary mechanism of extracting excess heat during refueling and heating storage bed during hydrogen release. The process of heat removal from a bulk of the storage bed is slow because it relies on the heat conduction through metal hydride powder, which is a poor heat conductor. Heat conduction is usually highly restricted through a powdery storage bed typical for metal hydrides and therefore results in slow charging of hydrogen in the metal hydride. In addition, the complex distribution lines for the cooling medium such as water or the like results in excess increase in the overall weight of the storage system. This additional weight reduces the effectiveness of hydrogen systems for use in vehicles. There is a need, therefore, for an improved system and method of hydrogenating and dehydrogenating a hydrogen storage vessel.